Truly random numbers are required in many branches of science and technology, from fundamental research in quantum mechanics to practical applications such as cryptography. While a pseudorandom number generator can expand a short random seed into a long train of apparent “random” bits using deterministic algorithms, the entropy of generated random numbers is still bounded by the original short random seed. To generate true randomness, researchers have been exploring various physical processes.
Quantum random number generation is an emerging technology, which can provide high-quality random numbers with proven randomness. Different from physical random number generators exploring chaotic behaviors of classical systems, a quantum random number generator (QRNG) harnesses the truly probabilistic nature of fundamental quantum processes.
In general, a conventional process of random number generation can be divided into two steps: the measurement step and the randomness extraction step. Conventionally, in the first step, attempts may be made to perform a measurement on an entropy source. In practice, both the source and the detection system are not perfect and will introduce technical noises in addition to any quantum noise. In the worst-case scenario, the technical noises could be accessible to (or even controlled by) a malicious adversary (Eve) and thus cannot be trusted. Furthermore, the raw output of the detector may not be uniformly distributed. The second step of the conventional process for random number generation is to perform randomness extraction. However, in practice, randomness extraction may be ineffective because the quantum noise is not dominant over the technical noises.
Among various QRNG implementations, schemes based on photonic technology have drawn a lot of attention for high rates, low cost, and the potential of chip-size integration. Both conventional single photon detectors and conventional optical homodyne detectors have been employed in photonic QRNGs. Conventional optical homodyne detectors provide highly efficient photo-diodes working at room temperature but are limited to exploring vacuum fluctuation and laser phase noise.
Nevertheless, there are still practical challenges in these conventional systems. In a QRNG based on vacuum noise, one major source of technical noises is the electrical noise of the homodyne detector. For instance, the electrical noise often interferes with measurement of the shot noise. This issue is considered to limit the operating speed of this type of QRNG. In a QRNG based on laser phase noise, fiber interferometers with large arm imbalance (on the order of nanoseconds) are often employed. To achieve high random number generation rates, either phase stabilization of the fiber interferometer or highspeed modulation of the laser source is utilized. In a more recent chip-size design, instead of using a cumbersome fiber interferometer, the outputs from two independent distributed feedback (DFB) lasers are mixed at a beam splitter. Random numbers are generated by operating one laser in a gain switching (GS) mode, while the other laser is in a continuous wave (CW) mode. Essentially, the laser in the GS mode provides a train of phase randomized laser pulses, while the laser in CW mode acts as a phase reference in coherent detection. To achieve a high interference visibility in this conventional system, sophisticated temperature control is utilized to match and stabilize the wavelengths of the two lasers.